Conventional semiconductor fabrication utilizes a scanner or other light source to form devices or features on the surface of a wafer. More specifically, a beam of light is directed at a portion of the wafer through a mask to cover a spatial area called the “shot” area. The shot area generally covers a group of dies on the wafer or a single die depending on the size of the die and the configuration of the mask. For the case of multiple dies within a shot, the shot area covers an array of die—for example, three die in one direction and two die in the perpendicular direction. This configuration is referred to as a ‘full shot.’ At the edge of the wafer where the full die or full shot pattern is not present, the shot area consists of a ‘partial shot’ where only the actual die printed on the wafer are exposed, not the complete ‘full shot’. Sometimes partial die at the wafer edge are also printed as part of the shot. Although a wafer has a generally horizontal surface on which the features are formed, the edge of the wafer is typically rounded such that the wafer has a substantially rounded profile. Typically, the light source is operated at a standard exposure intensity for all devices and features formed on the surface of the wafer regardless of the location of the particular devices and features on the surface of the wafer. This results in a situation in which the same exposure intensity is used near the center of the wafer and at the edge of the wafer. This standard exposure intensity, however, can result in deformities in the features formed near the rounded edge of the wafer, especially on partial edge shots. In particular, via holes with small dimension and high aspect ratio are difficult to pattern on the wafer edge dies due to wafer edge topography. Furthermore, if the edge dies use the same exposure as center dies, via holes formed around the edge can be either small in diameter or not open at all. To address this problem, a common practice is to employ a technique called ‘By-Shot-Exposure (BSE),’ in which the exposure is adjusted for each shot at different locations across a wafer.
By-Shot-Exposure (BSE) affords the ability to use higher exposure on edge shots than on center shots. However it does not permit localized exposure adjustment on dies within a shot. Some edge shot dies reside on the flat part of the wafer. These die should be printed using standard exposure. Although theoretically all dies can be exposed with different, optimized exposures, compromised scanner throughput makes this impractical.
Accordingly, there is a need to overcome these and other problems of the prior art to provide a new method for patterning holes at wafer edge.